Speed up pldexp_generic.

Rasmus Munk Larsenrequested to merge
rmlarsen1/eigen-new:ldexp into master

Speeds up pexp by up to 6%.

Measurements on SkylakeX:

SSE4.2:
name                      old cpu/op   new cpu/op   delta
BM_eigen_exp_float/1      1.88ns ± 1%  1.68ns ± 1%  -10.88%  (p=0.000 n=54+47)
BM_eigen_exp_float/8      28.9ns ± 1%  28.5ns ± 0%   -1.37%  (p=0.000 n=51+47)
BM_eigen_exp_float/64      145ns ± 1%   139ns ± 0%   -4.09%  (p=0.000 n=49+43)
BM_eigen_exp_float/512    1.11µs ± 1%  1.06µs ± 0%   -4.42%  (p=0.000 n=42+46)
BM_eigen_exp_float/4k     8.80µs ± 0%  8.40µs ± 0%   -4.54%  (p=0.000 n=42+42)
BM_eigen_exp_float/32k    70.2µs ± 0%  67.6µs ± 3%   -3.74%  (p=0.000 n=46+59)
BM_eigen_exp_float/256k    561µs ± 0%   537µs ± 1%   -4.27%  (p=0.000 n=45+45)
BM_eigen_exp_float/1M     2.24ms ± 0%  2.15ms ± 1%   -4.15%  (p=0.000 n=39+43)

AVX2:
name                      old cpu/op   new cpu/op   delta
BM_eigen_exp_float/1      1.70ns ± 6%  1.70ns ± 5%    ~     (p=0.488 n=60+60)
BM_eigen_exp_float/8      30.9ns ± 0%  30.9ns ± 0%    ~     (p=0.352 n=49+50)
BM_eigen_exp_float/64     84.1ns ± 4%  81.0ns ± 4%  -3.71%  (p=0.000 n=59+58)
BM_eigen_exp_float/512     520ns ± 4%   489ns ± 3%  -5.96%  (p=0.000 n=57+58)
BM_eigen_exp_float/4k     3.99µs ± 4%  3.77µs ± 4%  -5.45%  (p=0.000 n=48+46)
BM_eigen_exp_float/32k    31.8µs ± 5%  29.9µs ± 5%  -5.87%  (p=0.000 n=50+53)
BM_eigen_exp_float/256k    253µs ± 4%   239µs ± 4%  -5.65%  (p=0.000 n=50+53)
BM_eigen_exp_float/1M     1.01ms ± 4%  0.95ms ± 4%  -6.04%  (p=0.000 n=60+56)

AVX512:
name                      old cpu/op   new cpu/op   delta
BM_eigen_exp_float/1      2.64ns ± 1%  2.65ns ± 2%    ~     (p=0.061 n=51+54)
BM_eigen_exp_float/8      33.9ns ± 2%  33.9ns ± 2%    ~     (p=0.546 n=49+46)
BM_eigen_exp_float/64     88.5ns ± 3%  88.7ns ± 4%    ~     (p=0.703 n=57+59)
BM_eigen_exp_float/512     275ns ± 3%   274ns ± 3%  -0.60%  (p=0.009 n=52+54)
BM_eigen_exp_float/4k     1.77µs ± 3%  1.76µs ± 3%  -0.62%  (p=0.006 n=59+59)
BM_eigen_exp_float/32k    13.7µs ± 3%  13.7µs ± 4%    ~     (p=0.153 n=58+60)
BM_eigen_exp_float/256k    119µs ± 5%   118µs ± 4%    ~     (p=0.453 n=60+58)
BM_eigen_exp_float/1M      475µs ± 6%   475µs ± 5%    ~     (p=0.723 n=60+60)

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